Metalorganic chemical vapor deposition of layered structure oxides

ABSTRACT

A method of fabricating high quality layered structure oxide ferroelectric thin films. The deposition process is a chemical vapor deposition process involving chemical reaction between volatile metal organic compounds of various elements comprising the layered structure material to be deposited, with other gases in a reactor, to produce a nonvolatile solid that deposits on a suitably placed substrate such as a conducting, semiconducting, insulating, or complex integrated circuit substrate. The source materials for this process may include organometallic compounds such as alkyls, alkoxides, β-diketonates or metallocenes of each individual element comprising the layered structure material to be deposited and oxygen. Preferably, the reactor in which the deposition is done is either a hot wall or a cold wall reactor and the vapors are introduced into this reactor either through a set of bubblers or through a direct liquid injection system. The ferroelectric films can be used for device applications such as in capacitors, dielectric resonators, heat sensors, transducers, actuators, nonvolatile memories, optical waveguides and displays.

FIELD OF INVENTION

This invention relates to the field of ferroelectric layered structureoxides in the thin film form, and methods of fabricating the same usingmetalorganic chemical vapor deposition processes. Such materials haveuses in ferroelectric device applications such as capacitors,nonvolatile memories, sensors, displays, and transducers.

BACKGROUND OF THE INVENTION

Ferroelectric materials are characterized primarily by a spontaneouspolarization, the orientation of which can be reversed by an electricfield. In addition, these materials also display unique dielectric,pyroelectric, piezoelectric and electro-optic properties that areutilized for a variety of applications such as capacitors, dielectricresonators, heat sensors, transducers, actuators, nonvolatile memories,optical waveguides and displays. For device applications, however, it isuseful to fabricate ferroelectric materials in the form of thin films soas to exploit these properties and the design flexibility of thin filmgeometries.

Device applications also require that the bulk properties of theferroelectrics are achieved in the thin films and therefore it isnecessary to employ a deposition technique that can provide optimumthin-film characteristics such as stoichiometry, crystallinity, density,microstructure and crystallographic orientation. Although a variety ofdeposition techniques have been used, the growth of the films withcontrolled properties at relatively low temperatures is still achallenge and several techniques are being explored to achieve thisobjective. In general, thin film deposition techniques can be classifiedinto two major categories, i.e., (1) physical vapor deposition (PVD) and(2) chemical processes (see "The Materials Science of Thin Films",Milton Ohring, Academic Press, 1992; S. L. Swartz, IEEE Transactions onElectrical Insulation, 25(5), 1990, 935; S. B. Krupanidhi, J. Vac. Sci.Technol. A, 10(4), 1992, 1569). The chemical processes can further bedivided into two subgroups i.e., chemical vapor deposition and wetchemical processes including sol-gel and metalorganic decomposition(MOD). Among the PVD techniques, the most commonly used methods fordeposition of ferroelectric thin films are electron-beam evaporation, rfdiode sputtering, rf magnetron sputtering, dc magnetron sputtering, ionbeam sputtering, molecular beam epitaxy and laser ablation. PVDtechniques require a high vacuum, usually better than 10⁻⁵ Torr. Theadvantages of PVD techniques are (1) dry processing, (2) high purity andcleanliness, and (3) compatibility with semiconductor integrated circuitprocessing. However, these are offset by disadvantages such as (1) lowthroughput, (2) low deposition rate, (3) difficult stoichiometriccontrol, (4) high temperature post deposition annealing, and (5) highequipment cost.

The sol-gel and MOD processes for deposition of thin films are popularbecause of their simplicity. Additionally, they provide molecularhomogeneity, high throughput, excellent compositional control and lowcapital cost since no vacuum is required. However, for ferroelectricthin films they are limited by film integrity problems during postdeposition annealing and possible contamination problems that make themincompatible with semiconductor processing.

Of all the above mentioned techniques, metalorganic chemical vapordeposition technique (MOCVD) appears to be the most promising because itoffers the advantages of simplified apparatus, excellent filmuniformity, compositional control, high film densities, high depositionrates, excellent step coverage and amenability to large scaleprocessing. The excellent film step coverage offered by MOCVD cannot beequaled by any other technique. Purity, controllability, and precisionthat have been demonstrated by MOCVD are competitive with molecular beamepitaxy (MBE). More importantly, novel structures can be grown easilyand precisely. MOCVD is capable of producing an entire class of deviceswhich utilize either ultra-thin layers or atomically sharp interfaces.In addition, different compositions can be fabricated using the samesource.

Although MOCVD techniques are now being used to fabricate severaldemonstrative ferroelectric devices such as pyroelectric detectors,ultrasonic sensors, surface acoustic wave devices and severalelectro-optic devices, the primary impetus of recent activity inferroelectric thin films is the large demand for commercial nonvolatilememories. As mentioned earlier, ferroelectric materials arecharacterized by a spontaneous polarization that can be reversed byreversal of the applied field. The polarization in the material showshysteresis with the applied electric field; at zero field, there are twoequally stable states of polarization, +P_(r) or -P_(r), as shown inFIG. 1. This type of behavior enables a binary state device in the formof a ferroelectric capacitor (metal-ferroelectric-metal) that can bereversed electrically. Either of these two states could be encoded as`1` or `0` in a computer memory and since no external field (power) isrequired to maintain the state of the device, it can be considered anonvolatile memory device. To switch the state of the device, athreshold field (coercive field) greater than +E_(c) or -E_(c) isrequired. In order to reduce the required applied voltage, theferroelectric materials need to be processed in the form of thin films.Integration of ferroelectric thin film capacitors into the existing VLSIresults in a true nonvolatile random access memory device (see J. F.Scott and C. A. Paz de Araujo, Science, 246, (1989), 1400-1405 ). Inaddition to the nonvolatility, ferroelectric random access memories(FRAMs) also offer high switching speeds, low operating voltage (<5 V),wide operating temperature range and high radiation hardness.Furthermore, the ferroelectric thin films, electrodes and passivationlayers can be deposited in separate small facilities thereby obviatingthe need for any changes in the existing on-line Si or GaAs VLSIproduction. In principle, FRAMs could eventually replace static RAMs(SRAMs) in the cache memory, dynamic RAMs (DRAMs) in the main systemmemory and electrical erasable programmable read only memories (EEPROMs)in look up tables.

Although ferroelectric thin films offer great potential for nonvolatileRAMs, commercial usage has been hindered largely by serious degradationproblems such as fatigue, leakage current and aging that affect thelifetime of ferroelectric devices. A common source for these degradationproperties in oxide ferroelectrics is the presence of defects such asoxygen vacancies in the materials. Considering the problem of fatigue,ferroelectrics are noted to lose some of their polarizability as thepolarization state of ferroelectrics is repeatedly reversed. Fatigue(see I. K. Yoo and S. B. Desu, Mat. Sci. and Eng., B13, (1992), 319; I.K. Yoo and S. B. Desu, Phys. Stat. Sol., a133, (1992), 565; I. K. Yooand S. B. Desu, J. Int. Mat. Sys., 4, (1993), 490; S. B. Desu and I. K.Yoo, J. Electrochem. Soc., 140, (1993), L133) occurs because of both therelative movement of oxygen vacancies and their entrapment at theelectrode/ferroelectric interface (and/or at the grain boundaries anddomain boundaries). These defects are created during the processing offerroelectric films (with the desired ferroelectric phase) and can beclassified into intrinsic and extrinsic defects. Extrinsic defects arethe impurities that are incorporated in the films during processing andcan be controlled by controlling the processing environment. Intrinsicdefects can be divided into two types: (a) defects such as Schottkydefects that maintain stoichiometry and (b) defects that alterstoichiometry in the materials. Examples of the formation of thesedefects can be illustrated by considering the case of PbZr_(x) Ti_(1-x)O₃ (PZT) which is the most widely investigated ferroelectric thin filmmaterial for nonvolatile memory applications. Schottky defects inperovskite (ABO₃) ferroelectrics such as PZT may be represented by aquasi-chemical reaction (in Kroger-Vink notation) as:

    A.sub.A +B.sub.B +3O.sub.o →V.sub.A "+V.sub.B ""+3V.sub.o.sup.oo +A.sub.s +B.sub.s +3O.sub.s                               ( 1)

where A_(A), B_(B), and O_(o) represent respectively occupied A, B and Osites; V_(A) ", V_(B) "", and V_(o) ^(oo) represent vacancies of A, Band O atoms; and A_(s), B_(s), and O_(s) are the respective Schottkydefects. A typical example of defects that alter the stoichiometry arevacancies that are formed due to the vaporization of one or morevolatile components in multicomponent oxide materials. In the case ofPZT, for example, a processing temperature of at least 600° C. isrequired to form the ferroelectric perovskite phase. However, the PbOcomponent begins evaporating at temperatures as low as 550° C.,resulting in the formation of oxygen and lead vacancies as shown below:

    Pb.sub.Pb +Ti.sub.Ti +Zr.sub.Zr +3O.sub.o →xPbO+xV.sub.Pb "+xV.sub.o.sup.oo +(1-x)Pb.sub.Pb +Ti.sub.Ti +Zr.sub.Zr +(1-x) O.sub.o( 2)

Intrinsic defects may also be created by stresses developed in the filmsduring ferroelectric domain switching. It has been shown quantitatively[see S. B. Desu and I. K. Yoo, J. Electrochem. Soc., 140, (1993), L133]that relative migration of these oxygen vacancies and their entrapmentat the electrode/ferroelectric interface (and/or at grain boundaries anddomain boundaries) are important factors contributing to degradation inoxide ferroelectrics. The case of fatigue can be used to illustrate thispoint. As mentioned previously, fatigue in ferroelectric thin films isthe loss of switchable polarization with increasing number ofpolarization reversals. Under an externally applied a.c. field (requiredto cause polarization reversal), the oxygen vacancies have a tendency tomove towards the electrode/ferroelectric interface as a result of theinstability of the interface. Eventually, these defects are entrapped atthe interface and cause structural damage. This results in a loss ofpolarization in the material.

There are two possible solutions to overcome fatigue and otherdegradation problems. The first is to reduce the tendency for entrapmentby changing the nature of the electrode/ferroelectric interface.Multilayer electrode structures using ceramic electrodes such as RuO₂which minimize oxygen vacancy entrapment have been used to minimizefatigue problems in oxide ferroelectrics (see U.S. patent applicationSer. No. 08/104,861 for Multilayer Electrodes for Ferroelectric Devices,filed Aug. 10, 1993, the contents of which are hereby incorporated byreference). The second solution involves the control of defect density.The extrinsic point defect concentration may be minimized by reductionof impurity concentration or through compensation of impurities. La andNb doping are known to reduce the fatigue rate of PZT thin films on Ptelectrodes by compensating for the vacancies [see S. B. Desu, D. P.Vijay and I. K. Yoo, Mat. Res. Soc. Symp., 335, (1994), 53.]. Thestrategies for minimizing the intrinsic defect concentration may includechoosing compounds with inherently high defect formation energies orchoosing compounds that have no volatile components in their sublatticeexhibiting ferroelectric properties. Thus, another alternative toovercome fatigue and other degradation problems is to use aferroelectric compound that does not contain any volatile components inits sublattice that exhibits ferroelectric properties. This criterion issatisfied by many of the known layered structure ferroelectric oxides.

In the past, layered structure oxides have not been seriously consideredas candidates for ferroelectric device applications. Attempts were madeto use Bi₄ Ti₃ O₁₂, which is a layered structure material, as a gatematerial on a transistor in a switching memory application (see S. Y.Wu, IEEE Transactions on Electron Devices, August 1974, pp. 499-504).However, the device showed early degradation and was thereforeunsuitable for memory applications (S. Y. Wu, Ferroelectrics, 1976, Vol.11, pp. 379-383). It is believed that development of any successfulpractical devices using layered structure oxides has been hindered bythe inability to deposit high quality thin films of these materials.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a reliablemetalorganic chemical vapor deposition process of making high qualitylayered structure oxide ferroelectric thin films which are useful in theapplications of capacitors, nonvolatile memory devices, pyroelectricinfrared sensors, optical displays, optical switches, piezoelectrictransducers, and surface acoustic wave devices. It is a more specificobject of this invention to a provide a reliable deposition process ofmaking layered structure oxide ferroelectric thin films for overcomingdegradation problems such as fatigue, time dependent dielectricbreakdown and aging in nonvolatile memories. Preferably, the layeredstructure oxide materials are A_(n) Bi₃ Ti_(n+1) RO_(3n+9), ABi₂ R₂ O₉,Bi_(2n+2) Ti_(4-n) O_(12-n), A_(n+1) Bi₄ Ti_(n+4) O_(15+3n) where A=Ca,Pb, Sr, or Ba; and R=Nb or Ta; and n=0 or 1. (see E. C. SubbaRao, J.Phys. Chem. Solids, 23, (1962), 655; B. Aurivillius, Arkiv Kemi, 1,(1949), 463; E. C. SubbaRao, J. Chem. Phys., 34, (1961), 695; G. A.Smolenski, V. A. Isupov and A. I. Agranovskaya, Fiz Tverdogo Tela, 3,(1961), 895). These compounds have pseudo-tetragonal symmetry and thestructure is comprised of stacking of perovskite-like units between (Bi₂O₂)²⁺ layers along the pseudo-tetragonal c-axis. A large number of thesecompounds do not contain any volatile components in their sublatticethat exhibits spontaneous polarization. The tendency for formation ofdefects such as oxygen vacancies and thereby the degradation problemssuch as fatigue may thus be minimized.

The materials of the invention include all of the above materials pluscombinations and solid solutions of these materials. Preferably, thesource materials are alkyls, alkoxides, beta-diketonates or metallocenesof the corresponding elements of the layered structure oxide elements.Preferably, the substrate materials are Pt-coated silicon wafers(Pt/Ti/SiO₂ /Si), RuO_(x) -coated silicon wafers (RuO_(x) /SiO₂ /Si),sapphire, or MgO. Preferably, the metallization in the devices is Pt,MO_(x) (where M=Ru, Ir, Rh, Os etc.), YBCO (yttrium barium copperoxide), LSCO (lanthanum strontium cobaltate), Au, Pd, Al, or Ni.Preferably the ratio of Ta to Nb is about 0.4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical hysteresis loop of ferroelectric material.

FIG. 2 shows a schematic diagram of a typical ferroelectric capacitor

FIG. 3 shows a schematic diagram of a ferroelectric capacitor withbuffer layers.

FIG. 4 shows a schematic diagram of a hot wall MOCVD system for growingthe ferroelectric material in accordance with the present invention.

FIG. 5 shows a schematic diagram of a cold wall MOCVD system for growingthe ferroelectric material.

FIG. 6 shows a schematic diagram of the direct liquid injection systemfor vaporization of the source materials.

FIG. 7 shows hysteresis characteristics of Pt/SrBi₂ (Ta₀.8 Nb₁.2)O₉ /Ptcapacitors before fatigue cycling. The SBTN films were deposited bypulsed laser deposition.

FIG. 8 shows the fatigue behavior of Pt/SrBi₂ (Ta₀.8 Nb₁.2)O₉ /Ptcapacitors at 5 V and 1 MHz (bipolar square wave) frequency. The SBTNfilms were deposited by pulsed laser deposition.

FIG. 9 shows hysteresis characteristics of Pt/SrBi₂ (Ta₀.8 Nb₁.2)O₉ /Ptcapacitors after fatigue cycling. The SBTN films were deposited bypulsed laser deposition.

DETAILS OF A PREFERRED EMBODIMENT

Metalorganic chemical vapor deposition essentially involves chemicalreactions of a volatile compound of a material to be deposited toproduce a nonvolatile solid that deposits on a suitably placedsubstrate. The reaction in almost all MOCVD processes involvescopyrolysis of various organometallic compounds with other gases. Thisprocess is a variation of the basic chemical vapor deposition process;only the chemical nature of the precursors or starting materials aredifferent. As implied by the name of the process, the precursormaterials are metalorganic compounds. There are several variations ofthe basic MOCVD process. These may be broadly categorized as hot andcold wall processes based on how the temperature is maintained in thereactors, low pressure and atmospheric pressure processes, solid orliquid source delivery systems, and plasma or photo enhanced MOCVDprocesses.

The deposition of ferroelectric thin films using metalorganic vapordeposition methods has been performed in the past. However, most of thereported depositions are of perovskite type ferroelectrics such as leadtitanate and lead zirconate titanates (see U.S. patent application Ser.No. 07/999,738 for Ferroelectric Thin Films Made by MetalorganicChemical Vapor Deposition, filed Dec. 31, 1992; Ceram 014A; M. Okada, K.Tominaga, T. Araki, S. Katayama and Y. Sakashita, Japan. J. Appl. Phys.,29 (4), 1990, 718-722; B. S. Kwak, E. P. Boyd and A. Erbil, Appl. Phys.Lett. 53 (18), 1988, 1702-1704). The successful fabrication of layeredstructure ferroelectric films has not been accomplished using the MOCVDtechnique. The processes of the present invention in fabricatingferroelectric layered structure oxide films are demonstrated bydescribing a particular embodiment of the present invention in thecontext of fabrication of one particular ferroelectric device (i.e., aferroelectric capacitor for nonvolatile memory applications) using aparticular class of layered structure ferroelectric materials (i.e.,SrBi₂ Ta₂ O₉ -SrBi₂ Nb₂ O₉ solid solutions). It is emphasized that theparticular embodiments shown in the drawings and within thisspecification are for the purposes of example and should not beconstrued to limit the invention described later in the claims.

FIG. 2 shows a schematic of a ferroelectric capacitor in which theferroelectric material is layered structure oxide. The ferroelectriccapacitor is built on top of a substrate material 10 that may besilicon, a layer of silicon dioxide over a silicon chip, galliumarsenide, MgO, sapphire etc. Of course, the substrate 10 may be amultilayer structure having various circuit elements formed on a siliconchip having layers of silicon dioxide, polysilicon, implanted siliconlayers etc. to form a complex integrated circuit. A thin bottomelectrode layer 12 is deposited on top of the substrate by any of thestandard PVD processes or chemical processes of thin film depositionmentioned earlier. The bottom electrode materials may be metals such asPt, Au, Pt or Pd, conducting oxides such as MO_(x) (0<x<22), where M=Ru,Rh, Ir, Os or Re, conducting nitrides such as TiN and ZrN orsuperconducting oxides such as YBa₂ Cu₃ O_(7-x), Bi₂ Sr₂ Ca₂ Cu₃ O₁₀etc. If required, an intermediate adhesion layer (11) may be included toimprove the adhesion between the bottom electrode and the substratematerial. For example, in the case of Pt on Si/SiO₂ substrates, a thinTi interlayer is added to improve the adhesion between Pt and SiO₂. Theferroelectric material 13, which is a layered structure oxide, is thendeposited on the bottom electrode by the processes of the inventiondescribed later. The top electrode material is then either depositedthrough a shadow mask to form the electrodes of the required areadirectly or deposited completely over the ferroelectric films and lateretched after suitable masking using one of the standard VLSI etchingprocesses such as reactive ion etching, wet etching, ion milling, plasmaetching etc. to make several capacitors on a wafer. Once again, thematerials of the top electrode may either be the same as used for thebottom electrodes or any combination thereof. If required, buffer layers15 and 16 may be added between the ferroelectric layer 13 and theelectrodes 12 and 14 at the bottom and top, respectively, as shown inFIG. 3.

The selection of the precursors is the most critical step in MOCVD forsuccessful deposition of complex oxide films. The ideal precursors forMOCVD have to meet the requirements of high vapor pressure at lowvaporization temperatures, low decomposition temperatures, large"window" between vaporization and decomposition temperatures, nocontamination form organic constituents of the precursors, stabilityunder ambient conditions and nontoxicity. In the present invention,several types of metallorganic compounds have been used as precursors togrow the complex layered structure films. These include metal alkyls,metal alkoxides, metal β-didetonates and metallocenes. Most of themetaloxide precursors have reasonable vapor pressures at relatively lowtemperatures. In general, metal oxides and metal β-diketonates are lessvolatile than their alkyl equivalents, but they are easier to handle andmuch less toxic. One of the assets of metallorganic precursors is thattheir physical and chemical properties can be tailored by making smallchanges in their chemical structure. For example, the volatility of ametal β-diketonate can be increased by varying the alkyl or fluoroalkylgroup on the chelating ring.

A schematic of the hot wall MOCVD apparatus 40 used in the presentinvention is shown in FIG. 4. The reaction chamber 50 was a stainlesssteel tube having an inside diameter of 75 mm. The chamber 50 was pumpedto low pressures before deposition by a mechanical pump 52 and thepressure inside the chamber 50 was monitored by an MKS BARATRON pressuresensor 54. This sensor could monitor pressures in the range of 12 to10⁻³ Torr. The temperature inside the chamber 50 was maintained by aresistively heated three zone furnace 58. To accomplish the condensationof the products and/or any kind of unreacted reagents, a cold trap 60was placed between the reaction chamber 50 and the mechanical pump 52.The precursor material was packaged in a stainless steel bubbler 62 withtwo connections; one was connected to the reactor by a stainless steelline 64 with a manual valve 66 and the other was connected to a N₂ gascylinder 70 with a mass flow controller 72. Aluminum housed mantleheaters with temperature controllers were used to control thetemperatures of the precursors. The precursors were kept at the desiredtemperatures within ±5° C. during deposition. The paths between thefurnace and the precursor bubblers were heated by heating tapes to atemperature ranging from 200° to 300° C. to prevent condensation ordecomposition of the precursors. The apparatus of FIG. 4 shows severalbubblers with their associated lines, mass flow controllers and gascylinders.

A schematic of the cold-wall MOCVD apparatus 80 used in the presentinvention is shown in FIG. 5. In this reactor only the substrates wereheated to high temperatures while the walls of the reactor weremaintained at around 250° C. A three inch diameter substrate heater 82that could be operated at a maximum temperature of 900 ° C. wasemployed. The temperature of the substrate heater 82 was positioninsensitive within an 8° C. range, and the variation in the temperaturewith time throughout the growth process was within 1° C. A substrateholder 84, made of INCONEL and placed in direct contact with thesubstrate heater 82, was used to hold the substrates 86. The substratetemperature was monitored by a thermocouple 88 which was placed insidethe center of the substrate holder 84. The specimens were adhered on thesubstrate holder by silver paste. The silver paste was used to improvethe heat conduction and temperature uniformity of specimens. Thedistance between the inlet 90 of the source mixture and the substrates86 could be varied form 1.5 to 10 cm. The setup and control of thebypass line and the bubbler heaters were similar to those of thehot-wall apparatus. The bubblers used in the present invention have acylindrical shape 1.5 inches in diameter and 6 inches high.

With the use of conventional bubblers as shown in FIGS. 4 and 5, it maysometimes be difficult to obtain precise control of the stoichiometry,especially in complex multicomponent oxides. In many cases, theprecursor materials are solids which decompose at temperatures close tothe temperature at which they sublime. During the deposition, thebubbler is held at sufficiently high temperatures to sublime thereagents and consequently significant decomposition of the growthreagents may occur during deposition leading to variations in thecomposition and poor reproducibility between different runs. Also, itmay not always be possible to use volatile source reagents. To counterthese drawbacks, in the present invention, we have also incorporated amethod of liquid source delivery. In this method, the reagent source isheld in the form of a liquid (for solid sources, a solvent is added tomake it in the form of a liquid) and vaporized on a vaporization matrixstructure at an elevated temperature. The vapor source is thentransferred into the pyrolysis chamber of the MOCVD reactor by a carriergas that flows past the vaporization matrix. A schematic of the liquidsource delivery system used as a part of the present invention is shownin FIG. 6. The source materials of the desired thin film compound weremixed stoichiometrically and held in the liquid form in an Erlenmeyerflask 110. The source was transferred to the flash vaporization chamber102 by a Masterflex Economy drive 128 (basically a pump with liquid flowmeter) through a series of tubes as shown in FIG. 6. A needle valve 104was inserted in the flow line to control the flow of the liquid and wasconnected to the source end by silicone tubing 106 and the vaporizationchamber end by stainless steel tubing 108. The source was transferredfrom the flask 110 to the silicone tubing 106 through a glass rod. Thevaporization chamber 102 was sealed on the source end by a flange 114.The stainless steel tube 108 that provides the path for the liquidsource delivery was inserted into the vaporization chamber 102 through atight fit hole drilled into the flange 114. The other end of the chamber102 was connected to the pyrolysis chamber of the MOCVD reactor, thetemperature of which is controlled by a preheat chamber temperaturecontroller 132. The vaporization chamber 102 was heated as a whole andthe temperature was controlled using a temperature controller 118. Apreheated carrier gas (N₂) was used to transport the vaporized sourcefrom the vaporization chamber 102 to the pyrolysis chamber. The flowrate of the carrier gas was controlled using a mass flow controller 130.The carrier gas was sent through a preheat chamber 120 to heat thegases. This whole system is termed as direct liquid injection (DLI) andthe MOCVD process performed using this method of source vaporization isknown as DLI-MOCVD. See U.S. Pat. No. 5,204,314 by Kirlin.

For the deposition of the films, the substrates are loaded, the reactionchamber is sealed, and the system is evacuated to a desired pressure.The reaction chamber and/or substrate (hot wall and cold wall) are thenheated to the desired temperature. If the bubbler (FIGS. 4 and 5)configuration is used for vaporization of the source materials, then thesource bubblers are directly heated to the desired temperatures. IfDLI-MOCVD is the preferred process, the source liquid (stoichiometricmix of the individual precursors for each element in the final compound)is transferred to the vaporization chamber and vaporized at atemperature of around 250° C. on the evaporation matrix and carried tothe deposition chamber by a carrier N₂ gas also preheated to the sametemperature. At the start of the deposition, the source vaporsaccompanied by nitrogen carrier gas are sent through the bypass line;oxygen is sent through the reaction chamber to minimize the backdiffusion of source vapors into the chamber. The bypass process isperformed for about 3 minutes before beginning the deposition process.After the bypass process, the bypass valve is closed and the main valveof the reaction chamber is opened to start the deposition. The flow ofthe vapors from the bubblers or the DLI system is maintained for apredetermined deposition time. At the end of the deposition, thevaporization chamber and carrier gas are shut off, and the reactor isevacuated to base pressure before backfilling with air to atmosphericpressure. The samples are furnace-cooled below 100° C. before they wereremoved from the reactor.

The precursors for depositing the layered structure oxides can be one ofmetal alkyls, metal alkoxides, metal β-diketonates, and metallocenes.For example, the preferred precursors for depositing SrBi₂ (Ta_(x)Nb_(2-x))O₉ or BaBi₂ (Ta_(x) Nb_(2-x))O₉ are

Ba: Ba(thd)₂ (Ba-tetramethylheptadione),

Sr: Sr(thd)₂ (Sr-tetramethylheptadione),

Bi: Bi(thd)₃ (Bi-tetramethylheptadione),

Ta: Ta(OC₂ H₅)₅ (Ta-ethoxide),

Nb: Nb(OC₂ H₅)₅ (Nb-ethoxide).

If the DLI system is used, these precursors are mixed in stoichiometricratios in a solvent that is a 8:2:1 mixture in moles of tetrahydrofuran(C₄ H₈ O), iso-propanol (C₄ H₁₀ O), and tetraglyme (C₁₀ H₂₂ O₅). If theconventional bubbler method is used, the individual precursors arepacked into bubblers which are then heated to vaporize the precursors.

The superior ferroelectric properties of the layered structure oxidescan be illustrated by FIGS. 7-9. In this particular example, theferroelectric films shown in these figures are SrBi₂ (Ta₀.8 Nb₁.2)O₉deposited by pulsed laser deposition (PLD) method. The bottom and topelectrodes were sputtered Pt. FIG. 7 shows the hysteresischaracteristics of PLD SrBi₂ (Ta₀.8 Nb₁.2)O₉ films at an applied voltageof 5 V. The hysteresis loop is well saturated at this voltage and showsa Pr value of 11 μC/cm² and an Ec value of 65 KV/cm. The fatigue testson the films were performed using a 5 V square wave a.c. signal at afrequency of 1 MHz. FIG. 8 shows the result of the fatigue testperformed up to 10⁹ cycles. The films show very low fatigue rate. Thehysteresis properties on the films measured after cycling are shown inFIG. 9. The hysteresis properties of the films before and after cyclingwere similar. The leakage current density values for these films weremeasured as a function of applied voltage. At an applied voltage of 3 V,the films showed a leakage current density of 10⁻⁷ A/cm². The measuredresistivity of films were typically of the order of 10⁻¹² ohm-cm.

The particular embodiments described and shown in the drawings are forthe purposes of example and should not be construed to limit theinvention which will be described in the claims. For example, theprocess of invention can be used to fabricate high quality layeredstructure oxide thin films for ferroelectric nonvolatile random accessmemory applications. The process can also be applied to fabricate thesematerials for other applications such as piezoelectric, pyroelectric,electro-optic etc. The process may be applied to deposit these materialsonto structures (may be with different dimensions) other than thatdescribed specifically (capacitors) in the present invention. Theprocess may also be modified to add further processing steps. However,the basic inventive concept of the invention is still the same.

What is claimed is:
 1. A method of depositing ferroelectric layered structure oxide thin films on a substrate by metalorganic chemical vapor deposition, comprising steps of: maintaining said substrate at a reduced pressure in a chemical vapor deposition reactor; heating said substrate in the chemical vapor deposition reactor; vaporizing metalorganic precursors; transporting vapors of said precursors by a carrier gas or an oxidizing agent or both, and/or a diluent gas into said chemical vapor deposition reactor; and reacting the said vapors to form a thin film of the ferroelectric layered structure oxide on the said substrate, wherein the ferroelectric layered structure oxide is at least one of the compounds:A_(n) Bi₃ Ti_(n+1) RO_(3n+9) ABi₂ R₂ O₉ Bi_(2n+2) Ti_(4-n) O_(12-n) A_(n+1) Bi₄ Ti_(n+4) O_(15+3n) where A=Ca, Pb, Sr, or Ba; and R=Nb or Ta; and n=0 or
 1. 2. The method of claim 1 wherein only said substrate is heated and said reactor is maintained at a lower temperature.
 3. The method of claim 1, wherein the ratio of Ta to Nb is approximately 0.4.
 4. The method of claim 1 wherein said precursors are at least one of: alkyls of elements comprising said layered structure oxide film; alkoxides of elements comprising said layered structure oxide film; β-diketonates of elements comprising said layered structure oxide film; metallocenes of elements comprising said layered structure oxide film; or combination of at least two of alkyls, alkoxides, β-diketonates, and metallocenes of elements comprising said layered structure oxide film.
 5. The method of claim 1 wherein said precursors are Ba-tetramethylheptadione or Ba(thd)₂ for a barium-component, Sr-tetramethylheptadione or Sr(thd)₂ for a struontium-component, Bi- tetramethylheptadione or Bi(thd)₃ for a bismuth-component, Ta-ethoxide for a tantalum-component, and Nb-ethoxide for a niobium-component.
 6. The method of claim 1 wherein said substrate is at least one of: semiconductor composed of at least one of Si, SiO₂ -coated Si, or GaAs; a single crystal insulator composed of at least one of sapphire, ZrO₂, MgO, SrTiO₃, BaTiO₃, or PbTiO₃ ; or a complex integrated circuit.
 7. The method of claim 6 wherein said substrate is coated with a conducting material comprising at least one of: a metal electrode including Pt, Al, Au, or Pd; a conducting oxide electrode including MO_(x) (0<x<2) where M is at least one of Ru, Rh, Ir, Os, Re or lanthanum strontium cobaltate; a conducting nitride electrode including TiN or ZrN; and a superconducting oxide including YBa₂ Cu₂ O_(7-x) or Bi₂ Sr₂ Ca₂ Cu₃ O₁₀.
 8. The method of claim 7 where the said substrate and said coating are separated by an adhesion layer.
 9. The method of claim 1 further comprising heating said substrate to at least 450° C.
 10. The method of claim 1 wherein the films are deposited by heating said substrate to at least 450° C. and annealing at temperatures between 200° and 850° C.
 11. The method of claim 1 wherein the reduced pressure is from about 10⁻³ Torr to 760 Torr.
 12. The method of claim 1 wherein said precursors are vaporized at temperatures between 60° and 250° C. in bubblers.
 13. The method of claim 1 wherein the said precursors are held as liquids in a solvent and vaporized at temperatures between 60° and 250° C. on a vaporization matrix.
 14. The method of claim 13 wherein said solvent is a 8:2:1 mixture in moles of tetrahydrofuran (C₄ H₈ O), iso-propanol (C₄ H₁₀ O), and tetraglyme (C₁₀ H₂₂ O₅).
 15. The method of claim 1 where the oxidizing agent is at least one of oxygen and nitrous oxide.
 16. The method claim 1 wherein said oxidizing flows at between 200 and 2000 sccm into said chemical vapor deposition reactor.
 17. The method of claim 1 where the said carrier gas is at least one of nitrogen and an inert gas.
 18. The method of claim 1 where the said carrier gas and the mixture of the said carrier gas and the source vapors flows at between 1 and 2000 sccm into said chemical vapor deposition reactor.
 19. The method of claim 1 where the diluent gas is at least one of nitrogen and an inert gas.
 20. The method of claim 1 where the reacting of the vapors in the reactor is enhanced by a plasma.
 21. The method of claim 1 where the reacting of the vapors in the reactor is enhanced by a UV-lamp.
 22. The method of claim 1, further comprising manufacturing the deposited film into a ferroelectric device.
 23. The method of claim 1, further comprising manufacturing the deposited film into a nonvolatile memory device.
 24. The method of claim 1, further comprising manufacturing the deposited film into a capacitor device.
 25. The method of claim 1 wherein said ferroelectric material has a composition SrBi₂ (Ta_(x) Nb_(2-x))O₉ with x ranging from 0 to
 2. 26. The method of claim 1 wherein the said entire reactor is heated. 